Hi

I do not believe it was deleted by Mozilla, but withdrawn by the developer.

You can read more about this at:

https://www.mrfdev.com/enhancer-for-youtube

See also this Mozilla Discourse page that has a link to web.archive.org where you can find the extension

• https://discourse.mozilla.org/t/enhancer-for-youtube-is-gone/123749/17

author: Maxime RF
name: Enhancer for YouTube™ 2.0.121
compatibility: firefox: {min:68.0, max:*}
created: 2016-05-01T13:40:33Z
last_updated: 2023-08-25T15:43:11Z

Extensions like this can cause more problems than they are meant to solve if the developer cannot keep up with website changes.